The present invention relates to a switching power supply device that includes a boost power converter capable of correcting a power factor. More particularly, the invention relates to an active power-factor correction device (PFC device) circuit capable of correcting a power factor in response to power consumption in an external load.
In response to the global trend of energy savings, efforts to use energy more efficiently have been expended. To save energy, the IEC (the International Electrotechnical Commission) has defined a series of standards for power supply devices for use in electric appliances or equipment. The standards include the IEC-1000s related to harmonic components in a current in a power supply device. In addition, the environmental protection agency's (EPA's) specifications, the so-called ENERGY STAR, require to attain average power conversion efficiency of 84% or greater at each of output load factors of 25%, 50%, 75%, and 100%.
Power factor correction has come into wider use to solve problems related to harmonic components in a current in a power supply device, and a PFC circuit has been incorporated into a power supply device. FIG. 1 is a circuit diagram of a related switching power supply device that incorporates a PFC circuit. An AC input voltage supplied from an AC power supply 1 is rectified by a rectifier 2. The PFC circuit 3 shapes a wave form of an input current rectified in the rectifier 2 into a waveform which is proportional to that of an input voltage, thereby correcting a power factor, and suppressing harmonic components in the current. A DC-DC converter 4 converts a DC voltage supplied from the PFC circuit 3 into a high-frequency voltage by performing on-off switching using a switching element Q2 via a primary winding P1 in a transformer T1. Subsequently, the DC-DC converter 4 rectifies and smoothes the high-frequency voltage induced at a secondary winding S1 to a diode D2 and a capacitor C2, thereby converting into a DC voltage required by an appliance serving as a load 5. A control circuit 41 turns on and off the switching element Q2 so that an output of the DC-DC converter 4 (a voltage across the capacitor C2) attains a predetermined output voltage.
The PFC circuit 3 includes a first series circuit, a second series circuit, and a PFC control circuit 32. The first series circuit is connected to two ends of the rectifier 2, and includes a reactor L, the switching element Q1 having a MOSFET and the like, and a resistor R1. The second series circuit connects a drain and a source of the switching element Q1, and includes a diode D1 and a capacitor C1. Drive power for the PFC control circuit 32 is supplied from the DC-DC converter 4. Based on an AC input voltage supplied from the rectifier 2, an output voltage of the capacitor C1, and a voltage that depends on a current flowing through the resistor R1, the PFC control circuit 32 performs on-off switching of the switching element Q1 so that an AC input current is proportional to the AC input voltage and that the output voltage of the capacitor C1 attains a predetermined voltage.
FIG. 2 is a circuit diagram of the PFC control circuit of the switching power supply device shown in FIG. 1. The PFC control circuit 32 comprises an error amplifier 321, a multiplier 322, a current detector 323, a current error amplifier 324, and a PWM control circuit 325. An output voltage Vdc of the capacitor C1 is divided through resistors R4 and R5. The error amplifier 321 produces a signal for amplifying an error between the thus-obtained fraction of the voltage Vdc and a predetermined reference voltage Vr. An AC input voltage Vac supplied from the rectifier 2 is divided through resistors R2 and R3. The multiplier 322 multiplies the signal from the error amplifier 321 and the thus-obtained fraction of the AC input voltage Vac, thereby generating a target value of a reactor current of the reactor L. The current detector 323 detects a current flowing through the resistor R1, that is, a current flowing through the reactor L. The current error amplifier 324 compares the target value and an average current value provided from the current detector 323. The PWM (pulse-width modulation) control circuit 325 compares a result of a comparison outputted from the current error amplifier 324 and a sawtooth-waveform signal, and controls on-off operations of the switching element Q1 in accordance with the result of the comparison so that an AC input current that is proportional to both the AC input voltage and the control signal provided from the error amplifier 321 is obtained.
The PFC circuit 3 shown in FIG. 1 is of an active type, and continuously consumes electric power when the power supply device is supplied with an AC input from the AC power supply 1. Therefore, when an external load of the DC-DC converter 4 is light, the power supply device is decreased in power conversion efficiency. For example, an electrical appliance such as a television set which has a function of a standby mode is continuously used for a considerably long period of time in the standby mode while using only 10% or smaller amount of power of that used in a normal operation mode. Reduction in the power conversion efficiency at such a load condition is a problem in advancement energy saving in the same manner as does the conversion for a steady load.
Japanese Patent Application Laid-Open No. 2001-119956 has proposed a power supply device including a PFC circuit improved in conversion efficiency in a light load mode or a standby mode. The device includes a PFC circuit that actively controls power factor correction in response to an increase of power consumption in an external load. The above-mentioned power factor correction circuit comprises a converter that converts electric power and provides the power to a DC converter, a power factor correction control unit that controls power conversion by the converter, and a load detection unit. The load detection unit detects power consumption in the DC converter by an external load, and enables the power factor correction control unit when the power consumption exceeds a predetermined power level.